Regarding an MOT I've had the following.
I've had a fail when I had a 16" spare wheel fitted with 3 other of 17" wheels, I'd just had a puncture and due to the age of vehicle I left it on in case car was beyond repair.
My tourer last month passed with a matching pair of Bridgestones on the front, and the rear had one Uniroyal and one Roadstone which i had fitted when i had a puncture in the matching Uniroyal a while back.
The sizes are the same, not sure of speed rating but treads are certainly different.
My MOT tester is fanatical about tyres tread depth, and nearly every test has some warning about tyres wearing!! Didn't appear worried about tread patterns just condition of the tyres.
Incidentally I'm booked in tomorrow to have the uniroyal replaced with a new matching Roadstone, as it got an advisory for uneven wear due to worn bushes. Also having a full tracking check as I've just replaced the nearside rear suspension arms and bushes.